why we use LED drivers specifically is that they are constant current or constant voltage suppliers, and made specifically for that use. spending money on buy quality LED's and not using the proper power supplies is a waste of money.

You cannot use a PC power supply. They only put out 12v or 5V. You need a CC (Constant Current) supply and you need to forward bias the diode with a forward current of 700mA and voltage to turn on. Also a 12v supply will only be sufficient to power 4 LEDS.

As some have noted, LED drivers drive the LED with a constant current, meaning that the driver will fluctuate the output voltage to ensure the correct current is maintained while the resistance of the LED chances due to temp etc.